From mboxrd@z Thu Jan 1 00:00:00 1970 From: downing.nick@gmail.com (Nick Downing) Date: Tue, 14 Feb 2017 23:46:14 +1100 Subject: [TUHS] Another odd comment in V6 In-Reply-To: <0B8503D7-D696-4BB6-A2C7-75A95B603036@planet.nl> References: <378063CD-9B94-4AE3-B4BB-F41D2F6BFBF6@planet.nl> <0B8503D7-D696-4BB6-A2C7-75A95B603036@planet.nl> Message-ID: Yes, you are right, I had not paid attention to that pc=opc stuff, in fact 2.9 has a comment saying it's backing the PC up but the other BSDs do not, so I hadn't noticed that bit. :) I was probably thinking of another unix that implements it in the C library not the kernel, however it makes no difference conceptually. Interestingly, 2.11BSD has ERESTART defined as an errno and does the pc=opc thing if ERESTART was to have been returned as the errno. Whereas the other BSDs have another variable eosys which has just a few possible values, where one of those values (NORMALRETURN or some such) means that errno should be checked as well. I like the 2.11BSD way. V7 does not have the pc=opc thing and there is no mention of restarting, so I suppose EINTR just aborts the interrupted system call. cheers, Nick On Tue, Feb 14, 2017 at 11:27 PM, Paul Ruizendaal wrote: > Hi Nick, > > Many thanks for that background! > > I think the quote from the Gabriel paper indeed refers to software > interrupts, i.e. signals -- it would not make sense otherwise. The > ITS system that the MIT guy referred to is 'large', it ran on PDP10 > mainframes. > > I understand how executing a signal handler is piggy-backed on the > return from kernel mode. However, when the signal handler is > finished it could either continue with the next instruction or > re-excute the system call trap instruction. See > http://minnie.tuhs.org/cgi-bin/utree.pl?file=2.9BSD/usr/src/sys/sys/trap.c > (towards end) for details how this is actually done in 2.9BSD. > I think you referred to that mechanism as well. > > However, my question remains: why is that mysterious comment there, > above ttread() in V6, and is there a link with the Gabriel story? > > Paul > > On 14 Feb 2017, at 12:27 , Nick Downing wrote: > >> Well I don't know about this actual conversation in history so I can't >> help with that. But I can describe how interrupted system calls work. >> [..more..] >